Enterotoxigenic E. coli (ETEC) are a significant cause of morbidity and mortality world-wide, especially in children in developing countries. The World Health Organization estimates that the annual diarrhea burden due to ETEC in children less than five years of age in the developing world is approximately 210 million cases and 380,000 deaths. ETEC cause disease by colonizing the small intestine by means of colonization factors (CFs) and by production of either a heat-labile (LT) or heat-stable (ST) enterotoxin. The majority of clinical isolates produce both enterotoxins and the physiologic or immunologic consequences of simultaneous exposure to these two potent enterotoxins are unknown. Our preliminary data indicate that when LT and ST are both present, they work additively, increasing movement of water into the intestine over and above levels observed with either toxin alone. Our preliminary data also demonstrate that the levels of inflammatory cytokines produced by intestinal epithelial cells (IECs) in response to LT are significantly reduced in animals exposed to both enterotoxins. Based on our preliminary findings, our hypotheses for the proposed studies are 1) LT combined with ST increases fluid movement (diarrhea) when compared to either toxin alone by increasing cGMP, 2) the production of ST by LT/ST ETEC can significantly reduce the ability of the host to mount an effective innate immune response and 3) the production of ST by LT/ST ETEC can significantly reduce the ability of the host to mount an effective adaptive immune response (e.g., serum and mucosal antibody) against the infecting organisms (colonizing factors or LPS) or its heat-labile toxins. The specific aims are 1) Determine the role of LT/ST on accumulation of cyclic nucleotides in T84 cells; 2) Determine the role of LT/ST on innate immune responses in the murine intestine; and 3) Determine the role of LT/ST on adaptive immune responses to enterotoxigenic E. coli. At the conclusion of these studies, we will have a better understanding of the physiologic and immunologic consequences of simultaneous exposure to the two potent enterotoxins made by these organisms (LT and ST) and will have the information necessary to transition into non-human primate studies or human trials.